Complacent about a treatment for the gums (4)
I believe the answer is:
smug
'complacent' is the definition.
(thesaurus)
'a treatment for the gums' is the wordplay.
'a treatment for' indicates an anagram (I've seen this in other clues).
'gums' with letters rearranged gives 'SMUG'.
'about' acts as a link.
(Other definitions for smug that I've seen before include "Self-satisfied, stuffy" , "Like a stuffed shirt" , "Spruce" , "Arrogantly self-satisfied" , "Self-righteous" .)